The present invention relates to steam inlet valves and particularly relates to a combined control valve and stop valve constructed to provide optimum flow characteristics with a balance chamber and inlet bleed for enabling fast closure of the control valve.
In a typical steam turbine control valve, for example, of the type described and illustrated in U.S. Pat. No. 6,070,605, the disclosure of which is incorporated herein by reference, there is provided a steam inlet valve incorporating a control or interceptor valve and a main stop valve or reheat stop valve, hereafter called control valve and stop valve, respectively. The control valve is generally annular and has margins for seating against the annular valve seat in a closed condition of the control valve. A servo-valve typically positions the control valve relative to the valve seat to control the flow through the steam inlet. The stop valve includes a disk disposed within the annular space of the control valve and underlies the control valve.
As noted in the above-identified patent, the stop valve generally follows the movement of the control valve such that the sealing surfaces of the control valve and the underside of the stop valve provide a relative smooth flowpath for steam flow past the combined control and stop valves when the main stop valve is in an open condition. The principal purpose of the stop valve, however, is to close the steam inlet valve rapidly in response to a fast close signal to rapidly cut off the flow of steam through the steam inlet valve. The stop valve is controlled from below the inlet through a hydraulic cylinder, the fluid for which can be dumped quickly, enabling the stop valve to rapidly close against the seat. There is a need, however, for flow improvements to further reduce head losses and to facilitate fast closing of the control valve.
In accordance with a preferred embodiment of the present invention, there is provided a steam inlet valve for supplying steam to downstream steam machinery and wherein the valve has smooth continuous walls joining a steam outlet and valve seat free of vortices, thereby improving flow performance through the valve. Particularly, the outlet passage below the valve seat and in communication with the outlet of the steam inlet valve forms a smooth continuous transition with the valve seat, substantially maintaining the cross-sectional flow area. By this construction, an enlarged chamber below the valve seat is entirely eliminated. Moreover, the minimum changes in flow area reduce velocity head losses through the valve. In addition, an annular strainer is provided upstream of and about the control and stop valves, the margins of the strainer being recessed in grooves about the valve seat. In this manner, substantially only the strainer and none of its mounting structure is exposed in the flow of steam toward the valve seat which further eliminates abrupt changes in flow direction and consequent head losses.
In a further preferred embodiment of the present invention, there is provided a balance chamber above the control valve which has a bleed hole. When the control valve is signaled to fast-close the steam inlet, the bleed hole equalizes the pressure between the steam flowing past the valve and the steam in the balance chamber. This minimizes or eliminates a lowering of the pressure in the balance chamber which might otherwise tend to inhibit the movement of the control valve toward the valve seat. Additionally, buttress-type threads are interconnected between the control valve and the control valve stem. By using threads of this type, oxide buildup in service is unavoidably permitted, but the buildup is insufficient to cause the threads to bind and lock with one another. The valve is particularly useful for supplying steam to a steam turbine.
In a preferred embodiment according to the present invention, there is provided a valve for supplying steam to downstream machinery, comprising a generally annular valve seat defining a steam flow passage, a control valve in the passage including a control valve head having a recess on a downstream side of the control valve, the control valve head being movable toward and away from the valve seat, a stop valve having a stop valve head generally within the recess and a sealing surface for engagement with the valve seat in a stop valve closed position, an outlet passage below the valve seat and in communication with an outlet for directing the supply of steam to the machinery, the outlet passage and the valve seat having walls forming a smooth continuous transition therebetween without abrupt changes in flow direction for directing supplied steam past the seat, through the outlet passage and to the outlet substantially without formation of vortices to afford optimum steam flow characteristics with minimum losses.
In a further preferred embodiment according to the present invention, there is provided a valve for supplying steam to a steam turbine, comprising a generally annular valve seat defining a steam flow passage, a control valve in the passage including a control valve head having a recess on a downstream side of the control valve, the control valve head being movable toward and away from the valve seat, a stop valve having a stop valve head generally within the recess and a sealing surface for engagement with the valve seat in a stop valve closed position, an outlet passage below the valve seat and in communication with an outlet for directing the supply of steam to the turbine, a steam balance chamber above the control valve head defined in part by one side of the control valve head and which enlarges in response to movement of the control valve head toward the valve seat, and a bleed aperture in the control valve in communication with the balance chamber for precluding depressurization of the balance chamber in response to movement of the control valve head toward the valve seat.
In a further preferred embodiment according to the present invention, there is provided a valve for supplying steam to a steam turbine, comprising a generally annular valve seat defining a steam flow passage, a control valve in the passage including a control valve head having a recess on a downstream side of the control valve, the control valve head being movable toward and away from the valve seat, a stop valve having a stop valve head generally within the recess and a sealing surface for engagement with the valve seat in a stop valve closed position, an outlet passage below the valve seat and in communication with an outlet for directing the supply of steam to the turbine, including a valve stem coupled between the control valve head and an actuator for moving the control valve head toward the valve seat and buttress threads coupling the valve stem and the control valve head to one another.